1. Field of the Invention
The present invention relates generally to radio communications systems and radio communications control methods, and more particularly to a radio communications system and a radio communications control method including a radio base station, a radio relay station, and a mobile terminal.
2. Description of the Related Art
There are various known radio communications systems, of which W-CDMA has been in practical use for cellular phones. Further, WiMAX (Worldwide Interoperability for Microwave Access) has also been developed as a high-speed transmission system. Further, adaptive array antenna systems have been known as a technology for improving signal-to-interference-and-noise ratio (SINR) in the radio communications between a radio base station and a mobile or fixed terminal. These adaptive array antenna systems are roughly divided into those using beamforming that has the object of increasing signal power to a terminal and those using nullforming that has the object of reducing interference power to other terminals.
In the case of applying such adaptive array antenna systems, a radio base station needs to detect the existence of a terminal and its direction. It is common to use broadcast information (a broadcast message) in order to perform a network entry operation (terminal sensing) for such detection. It is assumed that this broadcast information can be received by all terminals in the area of the radio base station. Therefore, usually, the broadcast information is transmitted with an omnidirectional antenna. This is because if the position of a terminal is not determined, it is not possible to form a beam in the direction of the terminal. Since the omnidirectional antenna is smaller in gain than a directional antenna that forms a beam, the radius of its service area is reduced. Accordingly, terminals outside the service area of the omnidirectional antenna cannot receive communications services.
Therefore, the following two methods are proposed. According to the first method, with respect to the downlink, a radio base station repeatedly transmits broadcast information while a terminal calculates its time average to improve the SNR (signal-to-noise ratio) to a minimum acceptable level for reception, and receives the broadcast information. With respect to the uplink, the terminal also repeatedly transmits a response so as to notify the radio base station of its existence. On the other hand, according to the second method, with respect to the downlink, a radio base station preforms a beam and performs scanning while rotating the beam 360 degrees, and a terminal performs intermittent reception to receive broadcast information when the beam is oriented in the direction of the terminal. With respect to the uplink, the terminal repeatedly transmits a response so as to notify the radio base station of its existence the same as described above. In the case of applying either the first or the second method, a retransmission control sequence intervenes in a normal communications sequence. As a result, in either the first or the second method, it takes a long time before network entry, and the amount of processing increases in both the radio base station and the terminal. Further, neither the first nor the second method always ensures network entry, so that there also remains a problem in terms of service provision. To help this, regarding only the downlink, power boosting for broadcast information is well known but this increases the interference to other cells (this means it is not effective for SINR improvement, although effective for SNR improvement) and the problem for the power amplifier unit from a linearity perspective.
Further, a radio communications system having a radio relay station that relays transmission to and from a radio base station is known. The radio relay station is provided in order to cover the dead zone (holes) of the service area of the radio base station or to expand the service area of the radio base station. However, this results in addition of the link between the radio base station and the radio relay station and the link between the radio relay station and a terminal to the existing link between the radio base station and the terminal, thus reducing transmission efficiency. In particular, an increase in the number of links that go through the radio relay station results in a significant decrease in transmission efficiency. Therefore, the number of terminals that can communicate through the radio relay station is limited.
Further, according to a known configuration, a transmission array antenna, a transmission beam former, a reception array antenna, and a reception beam former are provided in a radio base station of a radio communications system to which W-CDMA is applied. Multiple incoming (uplink) beams are formed by processing by the reception beam former, and one of the incoming beams that maximizes reception power is selected to perform reception. Processing is performed by the transmission beam former so as to form a transmission beam in the direction of the selected incoming beam, and transmission is performed from the transmission array antenna. If no incoming (uplink) signal from a terminal is received for a predetermined period of time, the transmission beam former is controlled so as not to form a transmission beam toward the terminal. (See, for example, Japanese Laid-Open Patent Application No. 11-266228.) According to Japanese Laid-Open Patent Application No. 11-266228, multiple transmission and reception beams are preformed, and after recognizing a network entry request from a terminal, a beam is directed. This is based on the premise of fixed beam formation (so-called switched beam), and beam control adaptive to the movement of the terminal cannot be performed. Therefore, a handover or switching beams because of the movement of the terminal frequently occurs.
It is possible to expand the moving range of a terminal where communications are performable by providing a radio relay station in order to expand the service area or to cover the dead zone of the service area of a radio base station in a radio communications system and by applying an adaptive array antenna system to the radio base station. As described above, however, this results in a long period of time being required before network entry. It also causes the problem of reduction in transmission efficiency due to formation of the link between the radio relay station and the radio base station, the link between the radio relay station and the terminal, and the link between the radio base station and the terminal in the case of the terminal communicating with the radio base station through the radio relay station.